Many newer vehicles are equipped with an automatic stop-start system that automatically shuts down and restarts the vehicle engine to reduce fuel consumption. For example, the vehicle engine may be automatically turned off when the vehicle comes to a stop at a traffic signal, and automatically started when the driver releases the brake pedal. The stop-start systems typically require an auxiliary power source/battery to stabilize voltage during the autostart event. However, if the auxiliary power source/battery is disconnected, the missing auxiliary power source/battery may disrupt power to vehicle modules and prevent the autostart event. Heretofore, it has not been possible to detect an auxiliary power source disconnection without additional hardware. Accordingly, while such conventional stop-start systems work well for their intended purpose, it is desirable to provide an improved automatic stop-start system with auxiliary power source disconnect detection.